This invention relates to attenuation circuits such as are employed in automatic gain control (AGC) functions. Specifically, the present invention relates to attenuation circuits in which a signal is attenuated to a degree determined by a level of a control signal. More specifically, the present invention pertains to attenuation circuits which use a plurality of PIN diodes to effect the attenuation.
Conventional AGC circuits may incorporate attenuators which contain a plurality of PIN diodes arranged in "pi", "T", or "L" networks. The use of a plurality of PIN diodes widens the dynamic range of attenuation achievable over circuits which utilize only one PIN diode. Although a wide dynamic range represents a desirable feature, these AGC circuits often additionally exhibit one or more undesirable characteristics.
Some attenuator circuits have a structure in which a significant amount of current flows through portions of the attenuator which do not bias PIN diodes. As a result, such circuits consume a relatively high amount of power and may not be suitable for applications where circuits are powered by a battery.
Additionally, some attenuator circuits inhibit flexibility in adapting a given control signal to produce a desired attenuation curve. Many parameters characterize the manner in which attenuation may vary as a function of the control signal. For example, the range of control signal levels corresponding to an attenuation range, polarity of control signal changes relative to corresponding attenuation changes, and the amount of attenuation change corresponding to a given control signal level change are parameters which may not be independently controlled in such circuits. Accordingly, such circuits may not be suitable for applications which require precise control of attenuation by a control signal.
Further, other attenuator circuits exhibit parameters which excessively drift as the attenuators encounter temperature changes. These attenuators are not suitable for applications which require successful operation over a wide temperature range.